The invention relates to a shaft coupling for an axially slidable drive shaft.
Such a shaft coupling, which is known from DE 199 11 111 C1 for example, has an increased diameter cylindrical hollow tube end portion on the inner periphery of which axially parallel grooves are formed that serve to receive rolling members. An inner member of the cylindrical shaft coupling is disposed coaxially within said cylindrical hollow tube end portion, said inner member being provided, on the outer periphery thereof, with rolling member passages that are also oriented in an axially parallel manner and being connected, at the portion thereof pointing away from the tube end portion, to a constant velocity universal joint.
The inner grooves of the hollow cylindrical tube end portion and outer grooves of the coupling inner member are oriented toward each other in such a manner that they are facing each other for the purpose of bearing together the rolling members. The depth of the passage grooves for the rolling members and the diameter of the rolling members are configured in such a way that the rolling members hold the inner member and the outer member of the shaft coupling in a spaced-apart relationship on the one side and that they plunge so deeply into the passage grooves for the rolling members mentioned that the side walls of the grooves provide enough contact surface for transmitting a force to the rolling members and for leading said force out of them again.
With respect to an optimum force transmission between the driving and the driven shaft coupling component, this structural design is always a compromise because of the wish for axial slidability.
With respect to the force transmission from the driving shaft coupling member to the rolling members and with respect to said force being transmitted further from the rolling members to the driven shaft coupling member, only part of the force acting in the circumferential direction of the coupling inner member and of the coupling outer member can be transmitted so as to produce a driving effect in the known axially slidable shaft couplings as a result of these margin conditions. Because of the geometric situation described, another part of the forces applied to the rolling members does not act in the circumferential direction but in a more or less radially outward direction. This is not only detrimental to the efficiency of the torque transmission from the driving to the driven shaft coupling member but also causes a non negligible mechanical boring friction load to be exerted on the rolling member passages mentioned.
Beside the usual structural design of axially slidable shaft couplings described herein above, another axially slidable shaft coupling is known from U.S. Pat. No. 3,449,927 C1 in which a cylindrical coupling outer member has an approximately cruciform interior space in cross-section, said interior space serving to receive a rectangular coupling inner member. Passages for the rolling members are formed at the end of the branches of said cross, said passages being respectively disposed in the right and left corners of the branches and being held apart by spacers inserted in the grooves of the branches between the rolling members. The force is transmitted to the rolling member in such a manner that but one row of rolling members in each branch of the cross is used for transmitting the torque whereas the other row of rolling members remains mechanically unloaded. The drawbacks described herein above with respect to the efficiency of the torque transmission and to the boring friction load exerted on the rolling member passages are not overcome by this prior art either.
Another aspect in using axially slidable shaft couplings more specifically in motor vehicle propeller shafts is that these shafts rotate at very high speeds, often exceeding 10,000 revolutions per minute. These high speeds generate very high centrifugal forces on the rolling members of such type shaft couplings, adding to the mechanical load exerted on the rolling members and on the rolling member passages.